How to modify image in matlab and again store it?

I have got coloured 5 face images. I want to convert them to grayscale and store them in jpg format in the same folder.

I have used rgb2gray to convert it and then when i store it in the folder it gives the dimension of a colour image though the image appears black and white i.e my new image dimension after saving it in jpg become 480*640*3

How do I get rid of this 3 factor? Do i have to use adobe photoshop? or is it possible in matlab?